Curable coating compositions, especially thermoset coatings, are widely used in the coatings art. They are often used for topcoats in the automotive and industrial coatings industry.
High-gloss and color-plus-clear composite coatings are particularly useful as topcoats where exceptional gloss, depth of color, distinctness of image, or special metallic effects are desired. The automotive industry has made extensive use of these coatings for automotive body panels. These coatings require an extremely high degree of clarity and a low degree of visual aberrations at the surface of the coating in order to achieve desired visual effects such as a high distinctness of image (DOI).
As a result, high-gloss and composite color-plus-clear coatings are susceptible to a phenomenon known as environmental etch. Environmental etch manifests itself as spots or marks on or in the finish of the coating that often cannot be rubbed out. It can be difficult to predict the degree of resistance to environmental etch that a high gloss or color-plus-clear composite coating will exhibit. Many coating compositions known for their durability and/or weatherability when used in exterior paints, such as high-solids enamels, do not provide the desired level of resistance to environmental etch when used in high gloss coatings and color-plus-clear composite coatings.
Many compositions have been proposed for use as the clearcoat portion of color-plus-clear composite coating systems, such as polyurethanes, acid-epoxy systems and the like. However, many prior art systems suffer from disadvantages such as coatability problems, compatibility problems with the pigmented basecoat, and/or solubility problems. Moreover, very few one-pack coating compositions have been found that provide satisfactory resistance to environmental etch, especially in the demanding environment of automotive coatings.
It has been found that carbamate functional polymers such as those described in U.S. Pat. No. 5,356,669 can be used to provide coating compositions which exhibit significantly improved environmental etch resistance. Carbamate functional polymers have been used to provide commercially advantageous coatings compositions, especially as clearcoats in composite color-plus-clear coatings.
One method of producing carbamate-functional materials is by transcarbamylation or transesterification reaction of the hydroxyl-functional material with an alkyl carbamate (e.g., methyl carbamate, ethyl carbamate, or butyl carbamate). The reaction is carried out using a catalyst, such as an organometallic catalyst (e.g., dibutyl tin dilaurate). This method has certain disadvantages, one of which is that the presence of acid poisons the tin catalyst. If the carbamate material is produced by transcarbamylation and acid functionality is desired, then it is necessary to introduce the acid functionality after the transcarbamylation is complete. Another disadvantage is that the transcarbamylation process can require additional, expensive equipment to handle the low molecular weight carbamate compounds that are typically used in the process.
It would be desirable to make a wide variety of carbamate functional polymers from low cost and readily available reactants such as unsaturated linear anhydrides. Unfortunately, prior art methods for making addition polymers from linear unsaturated anhydride starting reactants typically require a purification step with respect to undesirable acid functional compounds resulting from the reaction of anhydride compounds, especially linear unsaturated anhydride compounds. Such purification steps normally involve the physical removal of such acid functional compounds and are often used in regards to monomers obtained from the reaction of an anhydride compound. Alternatively, the use of linear unsaturated anhydride compounds is often limited to processes for making addition polymers having high acid numbers.